Field of the Invention
Embodiments disclosed here generally relate to electrically conductive wellbore fluids. More specifically, embodiments disclosed herein relate to methods for creating or improving conductivity in wellbore fluids.
Background Art
The use of wireline well logs is well known in the art of drilling subterranean wells and in particular oil and gas wells. A wireline log is generated by lowering a logging tool down the well on a wireline. The tool is slowly brought back to the surface and the instruments on the logging tool take measurements that characterize the formations penetrated by the well in addition to other important properties of the well. For example, during logging wireline logs may use measurements of relative resistivity of the formation to determine geological composition of the downhole formation. Also, during drilling, such resistivity measurements may be useful to determine the location of the drill bit to enhance geosteering capabilities and directional drilling control. Thus, electrical logs and other wireline log techniques are depended upon in the oil and gas exploration industry to determine the nature of the geology and the reservoir properties of the petroleum bearing formations penetrated by the well, as well as other properties of the drilling process (e.g., the location of the drill bit). Further, wireline well logs are often the only record of the formations penetrated by the well available for correlation amongst different wells in a particular field.
When an electrical wireline log is made of a well, electrodes on the well logging tool are in contact with wellbore fluid or filter cake and hence the formation rocks through which the well has penetrated. An electrical circuit is created and the resistance and other electrical properties of the circuit may be measured while the logging tool is retracted from the well. The measurement of resistivity requires the presence of a highly conductive path between the logging tool and the formation (i.e., through the wellbore fluid). The resulting data is a measure of the electrical properties of the drilled formations verses the depth of the well. The results of such measurements may be interpreted to determine the presence or absence of petroleum or gas, the porosity of the formation rock, and other important properties of the well.
An alternative or supplement to wireline logging involves logging tools placed in specialized drill collar housing and run in the drill string near the bit. This technique is known as logging-while-drilling (LWD) or formation-evaluation-while-drilling (FEWD). Measurements such as electrical resistivity may be thereby taken and stored down hole for later retrieval during a “tripping out” of the drill string, or transmitted to the surface via mud-pulse telemetry. Such techniques are known to one of skill in the art of well drilling and subterranean well logging.
The use of oil-based muds and wellbore fluids has become increasingly popular since their introduction of the technology in the 1950's. Innovations in oil-based muds and wellbore fluids are of on-going importance with the development of environmentally friendly wellbore fluids and fluids having other special characteristics. Oil-based muds offer advantages over water-based muds in many drilling situations. In particular, oil-based muds are known in the art to provide excellent shale inhibition, borehole stability, lubricity, thermal stability, tolerance of contamination, and ease of maintenance. However, oil-based muds and wellbore fluids also have some disadvantages. One disadvantage is that normal resistivity and self potential measurements cannot be taken when the well has been drilled with a conventional oil-based mud or wellbore fluid due to the non-conductive nature of the oil-based wellbore fluids and muds.
Oil-based wellbore fluids, which may also be present as invert emulsion fluids, are water-in-oil emulsions in which oil is the continuous or external phase, and water is the internal phase. Invert emulsion fluids are often used because they provide superior drilling and wellbore stability performance. One disadvantage of oil-based wellbore fluids is that they are non-conductive in nature. Said another way, when invert emulsion fluids are used, any electrical path through the fluid is insulating due to the non-conductive nature of the external oil phase. This severely limits the amount and clarity of resistivity information that may be gathered from a wellbore using wireline logging.